Insulating system made of solid insulating material and impregnating resin

ABSTRACT

The invention relates generally to the field of insulating electrical conductors against partial discharge in the medium- and high-voltage ranges. In particular, the invention relates to an insulating system for an electric machine, in particular a rotating electric machine such as an electric motor and/or a generator. The invention provides for the first time a substitute for the conventionally used mica as a barrier material in an insulating system, such as the main insulation of rotating electric machines such as motors and/or generators. The substitute is based on a polyether-imide/siloxane copolymer, which can be processed two-dimensionally, for example by surface extrusion. In this way, sheets are produced and, after being processed in sheet form or as a laminate, can be used as planar insulating materials, or cut as strips, in insulating systems.

The invention relates in general to the field of the insulation of electrical conductors against partial discharge in the medium- and high-voltage range. In particular, the invention relates to an insulation system for an electric machine, in particular a rotating electric machine such as an electric motor and/or a generator.

Electric machines, such as for example motors and generators in the medium- and high-voltage range, include electrical conductors, a main insulation and a laminated stator core. The main insulation serves the purpose of electrically insulating the conductors with respect to one another, with respect to the laminated stator core and with respect to the environment. When the electric machine is in operation, electrical partial discharges can result in the formation of “treeing” channels in the main insulation. As a consequence of the “treeing” channels, electrical breakdown through the main insulation may occur. In the low-voltage range, where wires and cables are used, electrical discharges do not necessarily occur during operation, meaning that no barriers against partial discharges are required in that case.

In the present case, “medium- and high-voltage range” is understood to mean electrical energy technology that operates with a high voltage in the range above 700 V, up to and including 52 kV. This also encompasses insulation systems that are of interest for rapidly chargeable drive systems in the automotive industry.

A barrier in the form of a surface insulation material against partial discharges has to date been achieved mainly by the use of mica in the main insulation, the mica having a high partial discharge resistance. The mica is processed in the form of platelet-shaped mica particles with a conventional particle size of several hundred micrometers to several millimeters into a mica paper, which is then placed onto and adhesively bonded to a carrier, such as a glass fiber weave and/or insulation film, so that the mica particles produce the surface insulation material in the form of a mica short grain. A mica tape is cut from this mica short grain and is wrapped around the conductor to produce the main insulation. Subsequently, to produce the insulation system, the electrical insulation mica wrapping tape is impregnated with a liquid synthetic resin and the synthetic resin is then cured.

Known are insulation systems, such as for example the system known under the brand “Micalastic®”, in which the main insulation, comprising a mica wrapping tape as surface insulation material, is impregnated with a bisphenol epoxy resin in a vacuum pressure impregnation process.

Micalastic® is also known from EP2763142A1 and DE 102011083228A.

In order to improve the partial discharge resistance of the main insulation, it is known to use nanoscale particles which are dispersed in the synthetic resin prior to the impregnation. However, the presence of the particles results in a reduction in the pot life of the synthetic resin, which manifests in particular in a progressive polymerization of the synthetic resin prior to impregnation.

The production of the surface insulation material in the form of a mica short grain and/or a mica tape is complex and expensive.

In particular, on account of the requirements, mica-containing laminates comprising for example m-aramid and polyimide as carrier film have been hitherto used for traction motors for slot linings as well. Mica is a natural product and is mined in the form of mica schist. Accordingly, resources are limited, the mica is subject to fluctuations in quality depending on the mining location, is not always readily available and sourcing is associated with considerable costs, not to mention the complex processing for producing the mica tape as surface insulation material.

Accordingly, an object of the present invention is to provide a surface insulation material for the complete or partial replacement of the known mica paper-containing insulation materials for use in the production of an insulation system, in particular of the insulation system constituting the main insulation of an electric rotating machine such as a motor or a generator in the medium- and high-voltage range.

This object is achieved by the subject matter of the present invention, as disclosed in the description, the figures and the claims.

Therefore, the object is achieved by, and the subject matter of the present invention is, an insulation system, comprising a solid insulation material in the form of a surface insulation material and a synthetic resin, wherein the surface insulation material is a copolymer of a polyetherimide with a siloxane and the synthetic resin is a thermoset with which the surface insulation material is impregnated and subsequently cured in the form of an encapsulation.

The general finding of the invention is that in polyetherimide-siloxane copolymers there has been identified and demonstrated an enormous potential as an insulation material in the medium- and high-voltage range with respect to resistance against partial discharges. In particular, it has also been found that in the copolymers of polyetherimide and siloxane as a result of the less polar side groups of the siloxane with respect to the pure polyetherimide act as “impurity”, as a result of which the glass transition temperature falls. In addition, the siloxane content acts as a “plasticizer”, which is however chemically bound and cannot thermally withstand such high temperatures as the pure polyetherimide, but which is nevertheless suitable for relatively high temperature stresses. Polyetherimide-siloxane copolymers can be produced by suitable extrusion processes in sheetlike form as a film which for its part has sufficient elasticity to be used—in cut form—as wrapping tapes for wrapping tape main insulations.

The partial discharge resistance is evaluated via a surface profilometer by determining the specific erosion volume after electrical ageing. This is performed using the method of IEC 60343. The test structure and test conditions can be found in the publication: N. Müller; S. Lang; R. Moos: “Influence of ambient conditions on electrical partial discharge resistance of epoxy anhydride based polymers using IEC 60343 method”, Transactions on Dielectrics and Electrical Insulation 2019.

In an advantageous embodiment, the polyetherimide-siloxane copolymer is a block copolymer.

The content of siloxane in the copolymer is in the range from 0.1% by weight to 90% by weight, in particular 10% by weight to 60% by weight and in particular 20% by weight to 40% by weight, based on the total weight of the copolymer.

In an advantageous embodiment, the atomic proportion of silicon atoms in the copolymer is in the range from 1% to 25%, in particular 5% to 15%.

In an advantageous embodiment of the invention, the polyetherimide-siloxane copolymer is a block copolymer of general formula (I)

in which

-   -   R¹⁻⁶ are identical or different and are selected from the group         of         -   substituted or unsubstituted, saturated, unsaturated or             aromatic monocycles having 5 to 30 carbon atoms,         -   substituted or unsubstituted, saturated, unsaturated or             aromatic polycycles having 5 to 30 carbon atoms,         -   substituted or unsubstituted, saturated hydrocarbons having             1 to 30 carbon atoms,         -   substituted or unsubstituted, unsaturated hydrocarbons             having 2 to 30 carbon atoms;     -   V is a tetravalent linker group selected from the group of         -   substituted or unsubstituted, saturated, unsaturated or             aromatic monocycles and polycycles having 5 to 50 carbon             atoms,         -   substituted or unsubstituted, saturated hydrocarbons having             1 to 30 carbon atoms,         -   substituted or unsubstituted, unsaturated hydrocarbons             having 2 to 30 carbon atoms,         -   and also any combinations of linker groups comprising at             least one of the aforementioned groups;     -   g is 1 to 30 and     -   d is 2 to 20.

In a further advantageous embodiment of the invention, the copolymer may include one or more additives. By way of example, one or more metal oxides, such as for example TiO₂, Fe₂O₃ and/or MnFe₂O₄, and/or electrically nonconductive carbon-based fillers, such as for example industrial carbon black, may be used as additives.

The term “siloxane” is understood in the present case in principle to mean a compound having at least one —Si—O—Si— unit, in particular those that form in the polymer an —Si—O—Si— backbone as is common in silicones. For example, a polydialkylsiloxane, such as polydimethylsiloxane, or polydiarylsiloxane, such as polydiphenylsiloxane, are simple forms of a siloxane. Naturally, there are also mixed forms of siloxanes, such as for example a polyarylalkylsiloxane.

The term polyetherimide or “PEI” refers to a thermoplastic which is versatile in use since it is resistant to high temperatures and is classified as flame retardant, since it exhibits low smoke evolution even if it does still burn. PEI has a high strength and also high electrical breakdown resistance, low weight and is resistant to UV light and gamma rays. PEI is commercially available in particular as “ULTEM®”.

The impregnation resin used is a thermoset. For example polyester, formaldehyde, epoxide, novolak, silicone, polyesterimide, polyurethane, and any mixtures, blends and copolymers of the aforementioned compounds may be used. Impregnation resins for slot linings and/or wrapping tape insulations are generally known, inter alia from the patent specifications mentioned above. The solid insulation materials are impregnated with these impregnation resins and the resin is then cured in order to complete the insulation system.

A polyetherimide-siloxane copolymer is available under the trade name “Siltem™”, and has already been successfully used and tested. Siltem is an amorphous thermoplastic polyetherimide-siloxane copolymer and combines the temperature resistance of PEI with the flexibility of a silicone elastomer.

FIGS. 1 and 2 show the surface of two test specimens comprising insulation systems, in each case illustrating a solid surface insulation material impregnated with a synthetic resin that has been cured after performing impregnation. Both figures show the test specimen after electrical ageing. FIG. 1 illustrates the erosion of the insulation system produced with pure polyetherimide and FIG. 2 illustrates the erosion of the insulation system produced with the polyetherimide-siloxane copolymer of the invention in the form of a solid surface insulation material under the same conditions.

The defined standard test conditions for electrical ageing according to IEC 60343 are:

Voltage: 10 kV

Atmosphere: air 50% RH

Temperature: Room temperature, approx. 23° C.

Test duration: 100 hours

Flow rate: 1000 l*h⁻¹

Underneath FIG. 1 there is the key, where it can be seen that in FIG. 1 , for the insulation system with pure PEI, under the abovementioned conditions a circle forms around the centrally arranged conductor with an erosion depth, caused by partial discharges, of up to 80 μm, whereas, under the same conditions, the test specimen of FIG. 2 , with the insulation system that is produced identically except for the solid insulation material and comprises the copolymer according to the invention as solid insulation material, in the case tested the commercial product Siltem® and/or Ultem® STM 1600 as PEI-siloxane copolymer, also exhibits a circular ageing, but merely with an erosion depth of between −1 μm and −8 μm.

According to these tests, the present invention delivers a quantum leap in insulation technology, since here for the first time the complex-to-produce and costly mica-containing insulation material can be dispensed with.

It can be seen that compared to the pure polyetherimide the copolymer brings about an enormous increase, indeed a virtually complete resistance to partial discharge.

On account of the ascertained partial discharge resistance, the polyetherimide-siloxane copolymer, provided here for the first time as a mica substitute, is suitable as surface insulation material both for wrapping tape insulations and for sheetlike, for example slot lining, insulations, particularly in the use of motors, both for traction and as drive motor, but also for generators such as for example a wind power generator. Its exceptional elongation properties broadens the design scope of—for example—traction motors.

It is thus an achievable aim to produce both the m-aramid-containing slot linings and also the polyimide-containing insulation tapes with the surface insulation material of the invention made from polyetherimide-siloxane copolymer, without having to make tradeoffs in terms of the power density of the motors or generators. In particular, it is possible in both insulation systems to replace the mica paper and/or mica tape, which each—at least—comprise mica on a carrier, such as for example glass weave, and a tape adhesive for bonding the mica platelets, with the polyetherimide-siloxane copolymer, which inter alia can be processed by surface extrusion.

A polyetherimide-siloxane film produced for example by surface extrusion insulates for example the coils and/or the wires of the winding of an electric motor. These coils are then inserted into the slots of a laminated core and then impregnated with an impregnation resin, such as for example a polyesterimide and/or a silicone.

An insulation system according to an embodiment of the present invention for example comprises laminate with one or more films of polyetherimide-siloxane copolymer, also processed for example to give laminates with carriers and/or protective films—bonded for example to m-aramid or polyimide as carrier film.

The term “film” is understood in the present case to mean a sheetlike layer of a material. The film is a layer and not a layer stack.

In contrast, a “laminate” is generally a layer stack comprising one or more films. The layers may lie on top of one another in a full-surface manner—that is to say all layers are films—or in a partial-surface manner—that is to say at least one layer has for example a lattice structure and/or randomly distributed fibers and/or grid structure. It may also suffice for laminate formation for a film to be bonded with a weave or a laid scrim, for example a glass fiber laid scrim.

In the present case, a “laminate” is understood to mean a stack and/or a composite of at least two layers or films, that is to say for example at least one carrier and/or protective film, for example made of m-aramid or polyimide, with at least one film made of the polyetherimide-siloxane copolymer.

In particular in the case of slot linings, as are found for example in electric motors, wind generators, etc., single films of polyetherimide-siloxane copolymer as surface insulation material may tear, and therefore it is better here to use laminates having relatively tear-resistant films for the use of the polyetherimide-siloxane copolymer as insulation.

In a further embodiment, the laminates are for example cut into tapes and used in insulation systems.

In this way, an insulation of a slot for an electric motor may be protected in its entire length also and/or additionally by a surface insulation material made of polyetherimide-siloxane copolymer in a large thickness and/or processed as a laminate, that is to say for example in a composite with for example m-aramid films and/or polyimide films, as slot lining.

The winding is then inserted into the slots and the whole winding is in turn impregnated with an impregnation resin such as polyesterimide or silicone.

When producing the surface insulation material as wrapping tape, in particular a tape film is produced for the purpose mentioned here in a thickness in the range from 20 μm to 300 μm, in particular from 25 μm to 200 μm and very preferably in the range from 30 μm to 170 μm. A wrapping tape, for producing the solid portion of a wrapping tape insulation, is then produced from the tape film and is then impregnated with impregnation resin.

When producing the surface insulation material, for example for slot lining, in particular a film is produced for the purpose mentioned here in a thickness in the range from 12.5 μm to 500 μm, in particular from 25 μm to 450 μm and very preferably in the range from 50 μm to 300 μm. A surface insulation material is then produced from the film, for example by laminating a plurality of films, papers or films of different materials, such as m-aramid films or polyimide films, to produce the solid portion of a slot insulation system, and is then impregnated with impregnation resin.

Further advantages of the use of a polyetherimide-siloxane copolymer as surface insulation material are for example that

-   -   the entire insulation system can be produced much more favorably         than with mica-based surface insulation material,     -   the surface insulation material is thermally resilient from         approx. 150° C. to 200° C.,     -   the polyetherimide-siloxane copolymer is also flexible by virtue         of its siloxane content, meaning that it can be used as wrapping         tape,     -   electrically, it durably withstands—as tests have shown—the         required field strengths, this is because—as has been found in         the present case—if in the case of electric field strengths of         up to a maximum of 15 kV/mm (!) electrical discharges strike a         siloxane or an SiO₂ nanoparticle, a vitrified protective layer         forms which significantly increases the lifetime of an electric         rotating machine insulated therewith. The vitrified layer thus         formed can be readily detected by means of SEM, and in addition         elemental analysis by means of EDX is possible in order to         detect the silicon in the copolymer, and     -   it is partial discharge-resistant, as FIG. 2 of the present         description shows, which leads to a marked increase in the         electrical lifetime.

As a result of the invention, firstly, a replacement is provided for the conventionally used mica as barrier material in an insulation system such as the main insulation of electric rotating machines such as motors and/or generators. The replacement is based on a polyetherimide-siloxane copolymer which can be processed in sheet form, for example by surface extrusion. Films are produced that are processed in film form or else as laminate, cut as sheetlike insulation materials or as tapes, usable in insulation systems. 

1. An insulation system, comprising a solid insulation material in the form of a surface insulation material and a synthetic resin, wherein the surface insulation material is a copolymer of a polyetherimide with a siloxane and the synthetic resin is a thermoset with which the surface insulation material is impregnated and subsequently cured in the form of an encapsulation.
 2. The insulation system as claimed in claim 1, wherein the copolymer of polyetherimide and siloxane is a block copolymer.
 3. The insulation system as claimed in either of claims 1 and 2, wherein the copolymer has a siloxane content in the range from 0.1% by weight to 90% by weight, based on the total weight of the copolymer.
 4. The insulation system as claimed in any of the preceding claims, wherein the copolymer comprises an atomic proportion of silicon atoms in the range from 1% to 25%, based on all atoms in the copolymer.
 5. The insulation system as claimed in any of the preceding claims, wherein the copolymer is according to formula (I)

in which R¹⁻⁶ are identical or different and are selected from the group of substituted or unsubstituted, saturated, unsaturated or aromatic monocycles having 5 to 30 carbon atoms, substituted or unsubstituted, saturated, unsaturated or aromatic polycycles having 5 to 30 carbon atoms, substituted or unsubstituted, saturated hydrocarbons having 1 to 30 carbon atoms, substituted or unsubstituted, unsaturated hydrocarbons having 2 to 30 carbon atoms; V is a tetravalent linker group selected from the group of substituted or unsubstituted, saturated, unsaturated or aromatic monocycles and polycycles having 5 to 50 carbon atoms, substituted or unsubstituted, saturated hydrocarbons having 1 to 30 carbon atoms, substituted or unsubstituted, unsaturated hydrocarbons having 2 to 30 carbon atoms, and also any combinations of linker groups comprising at least one of the aforementioned groups; g is 1 to 30 and d is 2 to
 20. 6. The insulation system as claimed in any of the preceding claims, wherein the surface insulation material includes one or more additives.
 7. The insulation system as claimed in any of the preceding claims, wherein the copolymer used is the product available under the trade name Siltem™.
 8. The insulation system as claimed in any of the preceding claims, comprising a surface insulation material made from polyetherimide-siloxane copolymer at least in the form of a laminate, a film, in the form of a tape and/or of a tape cut from a laminate.
 9. The use of a polyetherimide-siloxane copolymer as surface insulation material and/or as wrapping tape for an insulation system in the medium- and high-voltage range.
 10. The use of a polyetherimide-siloxane copolymer as surface insulation material and/or as wrapping tape in electric traction motors.
 11. The use of a polyetherimide-siloxane copolymer as surface insulation material and/or as wrapping tape in generators of steam and/or gas turbines.
 12. The use of a polyetherimide-siloxane copolymer as surface insulation material and/or as wrapping tape in wind generators.
 13. The use of a polyetherimide-siloxane copolymer as surface insulation material and/or as wrapping tape in electric drive motors.
 14. The use of a polyetherimide-siloxane copolymer as surface insulation material in the form of a film and/or a laminate as slot lining.
 15. The use of a polyetherimide-siloxane copolymer as surface insulation material in the form of a tape as wrapping tape in an electric rotating machine, a motor and/or a generator. 